The ethylene propylene diene rubber (EPDM) market is rapidly growing due in part to the wide range of uses available for this general purpose elastomer. Indeed, the worldwide capacity was estimated in 2011 to exceed 1,093 ktons per annum. EPDM rubbers typically have remarkable properties including excellent resistance to ozone, heat, and weather, and the ability to be highly extended by oil. Accordingly, EPDM rubbers are used in various applications such as automotive, electrical, construction, roofing, and high-performance hoses and belts.
EPDM rubbers are traditionally commercially produced using conventional Ziegler-Natta catalysts based on transition metals, such as V and Ti (znEPDM) znEPDM typically has long chain branching that may be due to cationic coupling of pendant double bonds or Ziegler polymerization through both double bonds of a diene. Long chain branching does not typically affect polymerization rate, but has a profound effect on molecular weight, molecular weight distribution (MWD), ethylene compositional distribution (CD), and processability. Accordingly, znEPDM tends to have broad MWD and CD. According to P. S. Ravishankar and N. R. Dharmarajan (Rubber World, December, 1998) another advantage of long chain branching is that, before vulcanization, the extruded EPDM compounds (oil-free formulations) used in electrical wire and cable applications show smooth surfaces rather than extrudates with coarse surfaces. However, a broader CD in znEPDM (that is, a broader interchain distribution of ethylene derived units) may result in undesirably higher crystallinity.
Currently, metallocene catalyst systems based on Zr, Ti, or Hf are attractive for EPDM production (mEPDM), due in part to the lower cost of production and significant emission reduction. These metallocene catalyst systems tend to have oligomeric methyl aluminoxane (methyl alumoxane, MAO) or compounds such as tri-n-butylammonium tetra(perfluorophenyl)borate or N,N-dimethylaniliniumtetra(perfluorophenyl)borate as a cocatalyst. The resulting ionic complexes are highly stereospecific, producing mEPDM rubbers with very little long chain branching, and a narrow MWD and CD. Although the narrow CD is desirable, the lack of long chain branching and the narrow MWD adversely affects the performance of mEPDMs, especially in processability, that is, mEPDMs generally need further improvement, particularly in shear thinning and green strength.
There is therefore a need for mEPDM rubbers with improved processability, while retaining the advantages of narrow CD. There is also a need for processes to produce such mEPDM rubbers, and for improved articles made using such mEPDM rubbers.